I added a fireplace in my Rockwood 2604. The entertainment center looked something like yours.
The space under the TV was empty except for the subwoofer, and was just deep enough to install a 7" deep electric fireplace. I removed the cabinet doors, trimmed some wood to create a wide enough opening, relocated the subwoofer and plugged the fireplace into the receptacle below the TV.
Not hard to do, but make sure you take measurements twice and pick the appropriate sized fireplace.
2015 Rockwood A128S
2010 Rockwood 2604
2014 Ford F-250 CC SB 2WD
2008 Toyota Sequoia 2WD